74
An ultrasonic Study on Gelation Process in Egg White Protein
(초음파에 의한 egg white 의 gel 화의 연구)
Jong-Rim Bae*
*Taegu University, Dep. of Physics
배 종 림*
ABSTRACT
Ultrasonic measurement was made at 3MHz m theegg whitegel to study the properties of the natural protein The gelation process caused by heat was studied by the change，n the veioc.ty and absorption. An increase in absorption and a Crease in velocity were observed with a time lapse at several aging temperatures ranging from 60 to 751 Hysteresis phenomenon was also observed by varying the aging tem^rature as a parameter. Th,s phenomenon ls associated w.th the memorizing effect of the network structure of prote.n .n the gel. It was demonstrated that the ultrasonic technique was useful for monitoring the gelation process.
요 약
:연단백질인 egg white의 臨화의 과정을 초음파측정에 의하여 연구하였다 Egg 输快는 6此에서 源화 하고, 欧
어떤 온도에서 3知8 즉정을 하면 ge화의 진행에 따라 흡수는 중가, 음속은 감소하였다. Egg white의 gel화는 6（虎와 75P에서 m게 진행하였고, egg white gel의 비가역성을 반영해서 음속과 흡수 모두 이력현상이 관측되었다.
I. INTRODUCTION
Egg white, containing mainly water (88%) and protein(10%)f has attracted the attention of bios
cientists as one of 하圮 typical substances of natural protein, and has been investigated by several means(1~3). Gelation of egg white has been 안udied by several techniques : viscosity measurements, electrophoresis, light scattering, and other ch emical
analyses이. These techniques have certain limitations -the sample can only be used for a single expe- riment, and many samples are required to observe the total gelation process. The optical measurement cannot be made in the gel region where the sample is opaque. The ultrasonic method, on the other hand, can monitor the total process from sol to gel nondestr니ctively in the same sample. Thus, th은 ultrasonic method may provide a valuable tool for investigating the gelation process.
In this paper, we reported the results at 3MHz ultrasonic absorption and velocity measurements
An ultrasonic Study on Gelation Process inEgg White Protein 75
which were made in egg white below and above the gelation temperature. We noted two phenom
ena. First, an increase in absorption and a decrease in velocity were observed with the gelation. Sec
ondly, a remarkable hysteresis effect was found in the velocity and absorption of egg white gel with respect to temperature.
H. EXPERIMENT
The specimen was egg white taken from 하k
eggs of white leghorn chickens which had been kept at 5,C for two day after laying. Egg white was carefully filtered with 1mm mesh screen, and only the thin portion was used as a sample after being degassed under vacuum. The sample volume
=required was 20ml which was taken from three eggs. We measured the ultrasonic absorption in the. sol state of the egg white and found no aging effect and no observable differences between the thin and the thick portions of egg white13). The experimential method used for this measurement is essentially the pulse-echo overlap method. This system is, however, partially modified in our la
boratory to give the absorption as well as the velocity. In the present study, this system was used to measure absorption and velocity at 3MHz in the temperature range from 10 to 75^.
Fig. 1 Block (.lia^r;rn of the a[)p)aratus.
Figure 1 illustrates the block diagram of the apparatus used in the present study. Two trans
duces arc of X-cut quartz with a fundamental frequency of 3MHz. The entire front face of the transducers is grounded. They have exactly same dimensions and are fixed on both sides of the cell of stainless steel with path length 62.04mm. The cell wall was made taking special care that the cell ends be parallel : therefore, no extra mechanism to adjust the parallelism between the quartz plates.
Ultrasonic pulse is excited by sharp de voltage rising up within a time 아｝orter than 10ns. The rf amplifier can be turned to 3MHz and pulse at the fundamental frequency is received and ob
served. Only the first arriving pulse and the second echo are made visible on CRTfTrio CS-2070].
Repetition rate of CW oscillator [Trio AG-203] is adjusted so that the two pluse are overlapped and matched up cycle for cycle. The period of the oscillator which is measured by counter [ Hewlett- Packared 5315A] gives a round-trip time of the pulse and yields phase velocity with the known value of path length. The absorption was obtained from 나】。ratio of the pulse amplitude between 나｝e first arriving pulse and 나比 second echo. The ratio of two amplitudes determined with a variable step attenuator [Tamagawa Electronics TRA-601D]
which allows amplitude adjustment with 0.1-dB accuracy. The waveform of the echo was deformed by the reflection at the half wavelength quartz pistes, and this deformation introduced apparent attenuation over the intrinsic absorption. This effect was strictly corrected by relative measurement made with water as a reference liquid and the additional loss was appropriately subtracted from the observed value. The water showed no relaxa
tion effect in the 3MHz frequency range, and the acoustic impedence of the water is close to 나lat of egg white. The calculation of the sound dif
fraction loss was also made by using the equation
76 I TheJournal ofthe AcousticalSociety of Korea, Vol. 9, No.6 (1990)
of Schoch'^6,numerically with an effective radius (5mm) of the source and f=3MHz. The relative sensitivities of the absorption and the velocity measurement were 1% and 0.001%, respectively.
This systemwas very handy and convenient for simultaneous measurements of the velocity and the absorption, and was suitable for the present experiment which sho비d have been done at many different temperatues. To investigate the aging effect in the gel state, a series of measurements was carried out at intervals of 5minutes after the temperature was set at 60, 62, 64, 66, 70, and 7 5P. The temperature was controlled within 1/5 g by immerging the cell in a thermostated bath
[Jeio Tech RC10V].
IB. RESULTS AND DISCUSSION
1. Ultrasonic Hysteresis
The study of aging effect of egg white gel showed a marked increase in absorption and slight decrease in velocity at 6()〜754以 Figure 2 shows
a typical example of the time variation of ab
sorption and velocity observed at 75P. The temper
ature was raised from 70 P at the rate of 0.2 X) / min. The results are well fitted to the exponentiab curves represented by the solid lines. The time constant was 220( ±20) minutes for both corves.
The time constants obtained at different temper
atures agree well with each other as shown in Fig.
3. This value was found to be independent of the
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HZ
<H WZ OO
뿦 一H
Fig. 3. Time constant of the variation ofFig. 2.
Thecharacteristic time is 220 min.
Fig. 2. Ultrasonic absorption( • ) and velocity( O ) in egg white g이 observed in the aging experiment at 751?. Thesolid lines repersent the exponential curves fitted to the expenmential values
An ultrasonic Study on Gelation Process inEgg White Protein 77
aging temperature, while it may have been de
pendent on the rate of temperature rise.
The first stage of the experiment started in the sol state from 1(凡 and on. Then, the sample was heated up to 601? which is the gelation point, and kept at a constant temperature for 7 hours while the aging experiment was conducted, This proce
dure is indicated by a->b* in Fig. 4. The asterisk denotes the aging experiment. The data was taken periodically for six or seven hours until the ab
sorption and velocity almost reached the saturation values. The sample was then cooled down to 10 and the measurements were repeated in the gel state during increasing temperature from 10 to 60*0. The second aging experiment w거s made at 62*0 as expressed by c-*b^d*. The subsequent experimental procedure is represented by f
*
, and
The results of these experiments are summ
arized in Figs.4 and 5 for the absorption and the velocity, respectively. The absorption decreased with increasing temperature in the sol state as shown by the closed stars. This decrease reflects a natural trend common to every solution of protein as well as other molecules<6-7). The absorption began to increase rapidly upon gelation. The closed triangles denote the final values in the aging experiment.
Other symbols represent the values in the gel aged at different temperatures. At temperatures of 62, 64, and 66no gelation process was observed.
The absorption showed reversible change with temperature along the curve b->c, unless the tem
perature was raised newly beyond the aging tem
perature of the gel. The hystersis phenomenon w저s thus observed with respect to temperature : the absorption depended not only on the temperature at that time but also on the highest temperature the gel has experienced. A quite simlar hysteresis was observed also in the case of velocity. It should be noted, however, that the velocity maximum
"
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oo ma
300
0 20 40 60 SO
TEMPERATURE(°C)
4. Hysteresis of ultrasonic absorption. The closed stars indicate the experiment vahies in the sol state. The colsed triangles denote the final vahies in the aging experiment. The symbols
O, x, △, • and 口 represent the values ob- tamed after aging at 60, 62, 64, 70 and 75X3, respectively.
1562
15S0
1578
1576 1 5S4
1574
50 60 70 80
TEMPERATURE (°C)
Fig. 5. Hysteresis of ultrasonic velocity. All symbols have the samemeaning asthose in Fig. 4.
78 The Journal of the AcousticalSociety of Korea, Vol. 9, No.6 (1990)
in Fig. 5 is not associated with the gelation. The velocity in water, which is the major composition of the sample, had a maximum value at 74 °C⑻ and this peak was shifted down by the effect of protein molecules as a solute. The temperature of the velocity maximum in the gel state was 661!.
These hysteresis phenomena were explained in terms of the molecular aggregation in a gel. Above the gelation temperature, protein molecules in egg white were denatured and aggregated to make a network structure⑷⑼，which caused the change in the absorption and the velocity. If the gel was at a certain temperature Tm for a sufficiently long time, the network formation continued until it arrives at the final equilibrium state at Tm. This process was monitored through the ultrasonic properties as shown in Fig.2. The final state of aggregation is retained stable as long as the tem
perature is kept b시ow Tm, and therefore the absorption and the velocity show the reversible change. The first time the gel was heated up over Tm, aggregation still proceeded and a finer network was built. The network in gel became "harder”
and the ultrasonic properties had different values.
Thus, the thermal history strongly affected the values of absorption and velocity in the egg white gel.
2. G이ation Process
The total change in absorption and velocity in the aging experiment measured the network growth advanced during the temperature increase, and is plotted as a function of temperature in Figs.6 and 7. The bars indicate the differences between the final and initial values obtained when the tem- perature jwas raised. The open circles represent the accumulated values of the bars, and the solid lines drawn through the open circles are the estimated direct measurements of the degree of gelation at that LemperaL니re. A close resemblance was fo니nd
-1
60 65 70 7 5
TEIPErMTURE (°C)
Fig. 6. The total increase in absorption in the aging experiment (bars) and its accumulated values (circles). The latter givethe degree of gelation of egg white.
0
60 65 70 7 5
TEMPERATURE (°C)
Fig. 7. The total decrease in velocity in the aging experiment (bars) and its accumulated values
(circles). The latter give the degree of gelation of egg white.
Ta이e. 1. Composition of thin portion ofegg white.
Amount in thin egg white(%)
Molecular weight(g)
Water 88
Protein 1()
Ovalbumin 5.4 45,000 Conalbumin 1.3 80,000 Ovomucoid 1.1 28,0()()
Etc 2
An ultrasonic Study on Gelation Process in Egg White Protein 79
between Figs.6 and 7 excetp for the difference m signs. The change in the absorption and the velocity are no doubt of the same origin. The change is prominent at 60 and 75°C.
Table I shows main constituent the concen^
tration and molecular weight of the thin protion of egg white"01. The change at 70 and 75〔C in Figs.
and 7 may be attrib나ted to the aggregation of ovalbumin, the main constituent of egg white protein. Kato ef idiny has studied the agregation of ovalbumin in a diluted solution using a light scattering technique. They have reported that ovalbumin aggregated rapidly near 76t? where the thermal denaturation of the ovalbumin easily oc
curs. This agrees with our results. The ovomucoid is known not to be denatured 니nder common heating condition191. Therefore, the change at 601!
may be attributed to the aggregation of conalbu- min, the second major protein, which is known to be denatured at a lower temperature than ovalbumin19*. The protein molecules undergo heat- - induced denaturation and then these den거tured molecules aggregate with each other to form a network. The second process is possibly the rate determining step of the observed time dependence in Fig.2.
The mechanism of the increase in absorption and the decrease in velocity is not clear at the present time. The network-solvent interaction is likely to cause the additional absorption, as sug
gested by Gormally et #⑵.The decrease in velo
city is explained as a longitudinal modulus change.
The velocity is expressed by C=(M / p)x/2, where C is velocity, p is density in gel state, and M is longitudinal modulus. The density is not likely to change much with gel process. The longitudinal modulus is M=K+4/3xG, where K is bulk modulus and G is shear modulus. When egg white changes from a liguid to a gel state, the constru- cted network introduces a finite value of shear
modulus which may increase the longitudinal modulus113). However, the structure of the network would be very soft and the bulk modulus of its own is much smaller tan that of the solution. The overall longitudinal modulus would decrease as the network grows.
IV. SUMMARY AND CONCLUSION
Ultrasonic measurements were carried out to investigate gelation process in egg white at 3MHz by pulse-echo overlap which was partially modified in our labortory. The increase in absorption and the decrease in velocity caused by the gelation process of egg white were prominent at 60 and 75°C. The change at 60 and 75°C was attributed to the aggregation of conalbumin and ovalbumin, respectively. The increase in absorption was de
scribed by the interaction between the solvent and the network structure made of protien The decrease in velocity is explained as the longitudinal modulus decrease. Hysteresis phenomenon was observed by varying the aging temperature as a parameter. We have demonstrated that the ultrasonic technique was useful for monitoring the gelation process occurring in egg white.
ACKNOWLEDGMENT
This paper was supported in part by RE
SEARCH FUND for JUNIOR SCHOLARS. Korea Research Foundation.
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▲Jong Rim Bae was born in Kimhae, Korea, on December 19, 1953. He rec
eived the B.S. degree in physics in 1979 in Kyungsang National University, and the M.S. degree in applied physics in 1981 in National Fisheries University of Pusan. He passed an examination for study abroad by Korean Government Scholarship in 1983. He received the Ph.D. degree in applied physics, in 1988 in Tokyo University, Japan. Since 1988 he has been a ass
istant professor in the department of physics at Taegu University. His research interest is 니trasonic spectroscopy and its applications to materials sci
ence.
